1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) device, and more particularly, to a Liquid Crystal Display (LCD) device having contact holes in a zigzag (or oblique line) format on pad portions and/or varying the linewidths of the pad portions.
2. Discussion of the Background Art
A Flat Panel Display (FPD) having a light weight in a thin film shape is actively researched and commercialized for substituting a Cathode Ray Tube (CRT), a conventional display device, as interest and requirements in/for an information display and a portable information medium have increased in the 21st century.
In the FPDs, a Liquid Crystal Display (LCD) device is actively applied to a notebook or a desktop monitor since it implements excellent resolution, color display and definition, as a device for representing images using an optical anisotropy of liquid crystal.
Generally, the LCD device is configured by coupling two substrates on which a plurality of structures including Thin Film Transistors (TFTs) are disposed, to face each other through a series of processes and then sealing them by filling with liquid crystal therebetween. Here, on an array substrate that is a lower substrate between the two substrates, gate lines and data lines intersect with each other in a matrix format and TFT devices each electrically connected to the corresponding pixel electrode are provided in the intersecting region. Also, gate pads and data pads are respectively formed at each end of the gate lines and the data lines.
And, the gate pads and the data pads are connected to an operation circuit. The operation circuit is electrically connected to an external circuit to provide data input signals, and accordingly the data signals are separated from each other according to a control signal of the operation circuit itself and then transferred to each pixel. Here, the operation circuit and the data lines or the gate lines of the LCD device are connected to each other in a Tape Automated Bonding (TAB) method generally. Here, the TAB method refers to a method of mounting a package connected to an operation circuit onto a substrate.
FIG. 1 is a view showing a pad portion (top plan and side views) of a general LCD device.
As shown in FIG. 1, each pad portion connected to each end of the gate lines and data lines includes a pad metallic film 12, an insulation layer 17 and a transparent conductive film ITO (Indium Tin Oxide) 15. The insulation layer 17 may be implemented as a protection film 14 of a single layer or a double film of a gate insulation film 13 and the protection film 14. And, the pad metallic film 12 and the transparent conductive film ITO 15 contact each other at a pad open region (or contact hole) 16.
For example, the pad metallic film 12 formed of Chrome (Cr), for example, and the gate insulation film 13 and the protection film 14 may be formed over the pad metallic film 12. In addition, the pad open region 16 formed by partially etching the insulation film 13 so as to expose a portion of the pad metallic film 12 and the transparent conductive film ITO 15 compressed on the insulation film 13 so as to contact the pad metallic film 12 through the pad open region 16 are formed thereon in sequence. Here, the transparent conductive film ITO 15 is connected to a terminal such as an IC by the TAB method.
Accordingly, the pad portion having a lower pad layer provided with the pad metallic film 12 and an upper pad layer provided with the transparent conductive layer ITO 15 and connected to a TAB IC is provided.
However, due to an environmental regulation, the pad metallic film 12 is conventionally formed of Molybdenum (Mo) as a substitute for Cr. As a result, the pad metallic film 12 is mostly etched when forming the pad open region or contact hole on the pad portion formed of Mo. This causes a side contact between the transparent conductive film ITO 15 and the pad metallic film 12. The side contact means that a contacting area between the transparent conductive film ITO 15 and the pad metallic film 12 is remarkably small. Since a contact resistance of a general pad portion is determined by a portion where the transparent conductive film ITO 15 and the pad metallic film 12 contact each other, if the contact resistance is reduced due to the side contact, the properties of signals transferred to the gate or data pad portion from the TAB IC may be distorted.
In order to address this problem, a method for forming a plurality of pad open regions (contact holes) on one pad portion has been proposed, which will be discussed referring to FIG. 2.
FIG. 2 is a view showing a pad portion (top plan and side views) of a LCD device having a plurality of pad open regions and FIG. 3 is a planar view showing pad portions of a LCD device having multi-holes in a stripe format, in accordance with the related art. The pad portions parallel each other and the contact holes are aligned to each other as shown in FIG. 3.
As shown in FIG. 2, the pad portion includes a pad metallic film 22 formed of Mo on a transparent glass substrate 21 and a gate insulation film 23 formed on the pad metallic film 22. Here, though it is not shown, a gate line and a gate electrode are formed on the glass substrate 21 of a pixel region to be integral with the pad metallic film 22. In addition, a gate insulation film 23 is formed at a front surface of the substrate including the gate line and the gate electrode.
The pad metallic film 22 is formed of Mo in a sputtering manner. And, the gate insulation film 23 is formed of inorganic substance such as Silicon Nitride (SiNx) or Silicon Oxide (SiOx) in a PECVD (Plasma Enhanced Chemical Vapor Deposition) method.
A protection film 24 is formed on the gate insulation film 23 thus to form an insulation layer 27. The protection film 24 is formed of BCB (Benzocyclobutene) having a low permittivity.
Meanwhile, a data line and a source/drain electrode are formed on the gate insulation film 23 of the pixel region thus to form a Thin Film Transistor (TFT) together with the gate electrode, and the protection film 24 is formed at the front surface including the TFT.
By selectively removing the protection film 24 and the gate insulation film 23 of the pad portion, the pad metallic film 22 is partially or selectively etched and a plurality of substrate open regions 26 are formed.
And, a transparent conductive film ITO 25 over the protection film 24 so as to contact the pad metallic film 22 at the substrate open regions 26.
Accordingly, the pad metallic film 22 and the transparent conductive film ITO 25 contact each other through the plurality of substrate open regions 26 at the pad portion. This in turn reduces the contact resistance, which can minimize distortion of a signal being applied at the pad portion.
As another example of the pad portion having the multi-hole structure (open regions 26), substrate open regions 36 may be formed on the same line to be formed in the stripe format as shown in FIG. 3. Here, every pad portion 32 may have three or more substrate open regions 36.
However, in case that a photolithography process is applied to form the multi-holes in the stripe format in the related art, since a high-priced mask is used, a fabrication cost of the pad portions may increase.
In order to address this problem, a method for fabricating a LCD device to which a printing method is applied has been proposed according to the related art, which is discussed referring to FIG. 4.
FIG. 4 is a view partially showing a method for fabricating a LCD device to which a related art roll printing method is applied.
As shown in FIG. 4, when forming contact holes on a pad portion of the LCD device, a PR solution 45 ejected from a coater 30 is absorbed onto a roller 40 formed of polydimethylsiloxane (PDMS) and then the roller 40 is rolled on a cliché, namely, a printing plate 50 having embossed patterns 51 thus to form PR patterns 45a on the roller 40.
Then, the PR patterns 45a formed on the roller 40 are transferred onto a substrate 60 on which a pad metallic film and an insulation film 61 are mounted, and then contact holes (open regions) are formed by an etching process and a photo stripping process using the PR patterns 45a on the substrate 60.
Here, it is assumed that the pad metallic film is formed at the lower portion of the insulation film 61 on the substrate 60.
And, when forming these contact holes e.g., (open regions) on the pad portion, other contact holes are also formed on a protection film of a TFT portion, simultaneously.
However, the roll printing process is not limited to the formation of contact holes but can be broadly applied at a time of forming gate lines including a gate pad metallic film and data lines including a data pad metallic film.
Further, above all, when fabricating the printing plate 50 which is to be used to form the contact holes of the pad portion and the TFT portion formed on an array substrate of a LCD, it is difficult to form the embossed patterns 51 related to the formation of various contact holes, in consideration of a margin for obtaining a depth of the printing plate 50 in a wet etching process.
As a result, when a printing process is applied with using a photoresist, (substituting the photolithography process) in order to form the multi-holes on the pad portion in the stripe format as shown in FIG. 3 according to the related art, it is rather difficult to form these multi-holes on the pad portions.
As an example, FIG. 5A is a view showing a structure of an “ideal” printing plate having embossed patterns in an ideal stripe format, which can be used in the step of forming the multi-holes on the pad portions, and FIG. 5B is a section view showing a cut portion taken along line A-A′ in FIG. 5A. And, FIG. 6A is a view showing an example of a structure of an actual printing plate which is often used in the actual step of forming the multi-holes on the pad portions, and FIG. 6B is a section view showing a cut portion taken along line B-B′ in FIG. 6A.
In order to allow pads adjacent to each other to have contact holes along the same straight line (same as the photolithography process), the embossed patterns on a printing plate may be entirely formed in the stripe format as shown in FIG. 5A.
More specifically referring to FIGS. 5A and 5B, when performing the wet etching to form the embossed patterns 51 on the substrate 50, the photoresist (PR) coated on the glass substrate 50 is coupled to the glass substrate 50 with a small coupling force. Thus, the photoresist is partially stripped at the time of etching. Accordingly the embossed patterns 51, which are to be used to form the contact holes (e.g., open regions) on pad portions and other portions, are not precisely formed on the glass substrate 50.
To avoid the abovementioned limitation, when forming the embossed patterns on the glass/printing plate 50, as shown in FIG. 5B a mask layer is mounted on an initial glass substrate 50 and a photoresist (PR) formed thereon, the PR is exposed to the light and developed in consideration of the margin of the depth of the printing plate through the photolithography process, and a metallic layer is etched thus to form photoresist patterns 53 and mask patterns 52. Then the PR patterns 53 and mask patterns 52 are used to form the embossed patterns 51 on the plate 50.
Here, the wet etching for forming the embossed patterns 51 on the plate 50 is performed at the lower portion of the mask patterns 52 as shown in FIG. 5B. In the wet etching process, an isotropic etching phenomenon may occur where the embossed patterns 51 are etched in a round shape as the depth becomes greater and simultaneously a width becomes wider, in the wet etching process. Thus, the margin of the depth of the printing plate should be properly obtained.
Regarding this, in the related art when the etched depth of the printing plate 50 is approximately 40˜50 μm, it is capable of obtaining enough margin of the depth of the printing plate. Accordingly, when forming the mask patterns 52 for forming the embossed patterns 51 having 12 μm of each width in up and down and right and left, it is designed by additionally calculating 40 μm for each width in up and down and right and left.
However, it is assumed that a pitch between the pads (gate pads or data pads) is 60 μm and a contact hole having approximately 12 μm in width is formed on the pad portion having 26 μm in width according to the design of the data pad portion on the TFT array substrate. Then, if the printing plate has the margin of approximately 40 μm of depth as abovementioned, the embossed patterns which are to be used to form contact holes or open regions would not be formed properly. For example, as shown in FIGS. 6A and 6B, improper and undesirable embossed patterns 51a may be formed on a printing plate 50a using PR patterns 53a and mask patterns 52a. 
Hereafter, a problem or limitation generated when fabricating a TFT array substrate using the printing plate having the structure as shown in FIGS. 6A and 6B, will be discussed referring to FIGS. 7A-7C.
FIGS. 7A to 7C are views showing processes for fabricating a TFT array substrate using a printing plate 50a having the embossed patterns 51a as shown in FIGS. 6A and 6B according to a relate art.
As shown in FIG. 7A, when fabricating the TFT array substrate of a LCD device, the PR solution is not entirely or properly coated over the periphery of the pad portions of the TFT array substrate by the embossed patterns 51a of the printing plate after forming a protection film 73 due to the defects of the embossed patterns 51a discussed above. Accordingly a gate insulation film 71 formed on a substrate 70 and at both lower portions of a pad metallic film 72 formed of Mo is etched as well as the protection film 73, i.e., under cut phenomenon occurs, when performing a dry etching for forming the contact holes as shown in FIG. 7B.
As a result, when performing a rubbing process for the TFT array substrate, protruded portions at both sides of the pad metallic film 72 come off and then act as foreign substances that interfere with the other substrates/substances in the processes, which is a problem.